1. Field of the Invention
The present invention relates to a method for producing cubic boron nitride abrasive grains used for producing a grinding wheel, etc., and to cubic boron nitride abrasive grains.
2. Description of the Related Art
Cubic boron nitride is second to diamond in hardness and its chemical stability is higher than that of diamond. Thus, cubic boron nitride is increasingly employed as abrasive grains for producing grinding material, polishing material, or cutting material.
A variety of methods for producing cubic boron nitride have been proposed. Among them, best known and widely employed in the industrial field is a method in which hexagonal boron nitride is maintained in the presence of a substance such as solvents (also called as catalysts) under conditions where cubic boron nitride remains thermodynamically stable (approximately 4-6 GPa, approximately 1,400-1,600° C.), to thereby transform hexagonal boron nitride directly into cubic boron nitride (disclosed, for example, in Japanese Patent Publication (kokoku) Nos. 59-39362, 3-14495, 3-47132, and 3-15488).
The cubic boron nitride abrasive grains obtained through any of these methods have high hardness and chemical stability as mentioned above, and are employed in electroplated grinding wheels, metal-bonded grinding wheels, etc.
The cubic boron nitride abrasive grains obtained through any of the aforementioned methods are almost spherical (i.e., blocky abrasive grains). Thus, these abrasive grains are not suitably employed in grinding by means of a vitrified bonded grinding wheel which is required to have low grinding power.
Japanese Patent Application Laid-Open (kokai) No. 9-169971 discloses that “cubic boron nitride abrasive grains having a sharp shape and a relatively low level of defects” are used in order to enhance low grinding power of a grinding wheel employing cubic boron nitride abrasive grains, leading to “sustained low grinding power.” Although a grinding wheel employing cubic boron nitride abrasive grains produced through the above method attains improved grinding power as compared with a conventional grinding wheel employing blocky abrasive grains, grinding wheels of lower grinding power have been demanded in the industry.
The bond for use in the grinding surface of a vitrified bonded grinding wheel is melted to bind intergrain spaces during firing and provides strong bond strength after solidification through cooling. In order to attain excellent grinding power of a porous grinding wheel such as a vitrified bonded grinding wheel, porosity of the grinding wheel must be increased.
However, when the amount of a bond is reduced so as to increase porosity, retention of the abrasive grains is weakened, allowing falling of an increasing number of abrasive grains due to a load during grinding. Accordingly, the surface roughness of a work material deteriorates, and the interval between dressing operations is shortened, failing to attain a satisfactory grinding ratio. When the percentage of abrasive grain is reduced, the level of bridge structure among abrasive grains is lowered, to thereby decrease the hardness of the grinding wheel, even though retention of the abrasive grains is sufficient. Thus, the number of abrasive grains which cannot withstand the load imposed during grinding increases, resulting in falling of an increased number of abrasive grains and failing to attain a satisfactory grinding ratio.
In order to solve these problems, there is also proposed a method in which an aggregate is added so as to compensate for the reduction in abrasive particles or a bond. However, when an aggregate is added, the porosity of a grinding wheel decreases, thereby inhibiting enhancement of low grinding power.